The invention relates to a rotary actuator, and more particularly to a rotary actuator having a permanent magnet rotor capable of an angular movement through a given angle within a predetermined limit in a desired direction and which finds its applications in controlling the position of a needle or a cloth feeder of a sewing machine or driving a galvanometer or a pen recorder.
A rotary actuator is already known which operates on the principle of an indicator member of moving coil type. A known rotary actuator comprises a pair of spiral springs attached to and acting on the rotor which are balanced to determine a neutral position and which produce a control torque to counteract a drive torque when the rotor angularly moves in a selected direction from the neutral position. However, the rotary actuator employing a spring is disadvantageous in that a difference between the control torque and the driving torque at the maximum angular displacement of the rotor becomes large and the rotary actuator does not respond quickly. In addition, if a servo control of the rotor is desired, the rotor shaft should be coupled to the potentiometer shaft for cooperative operation, which however is difficult to be positioned with a high accuracy.